Ask the Stamping Expert: What's the best-in-class process for developing a progressive-stamping die?

Q: What is the best-in-class process for developing a new progressive-stamping die?

A: Many tool builders think the process of progressive-die development is undefined and difficult to predict, and therefore takes way too long.

Development is costly. It ties up engineering, design, toolroom supervision, toolmakers, a press, and possibly setup resources, all at virtually the same time. This can add up to thousands of dollars a day, not including the missed opportunity costs.

The following four guidelines will help you achieve best-in-class when it comes to minimizing development time and cost.

1. Design for Stability

In the November/December 2013 edition of STAMPING Journal®, I discussed a stamper’s dimensional problems—die shoe too thin; guide pins too small; no support under the shoe, resulting in flexing; down stops not robust enough to prevent hit-related variation - - and attributed them to a weak design. Two features that had a critical location relative to each other were done many pitches away from each other.

You must look at the part print and design the tool so that the way the cuts and forms are laid out will make the size and location of features a given. For instance, with a compound die, all inside and outside part features are functions of the punch and die. If the tooling is made to print, no development is needed. All features are guaranteed to be 100 percent relative by design.

Features that are tied together should be stamped at the same pitch. If you cannot do this because of the part geometry, you need to get creative. In my shop, we have added an extra tab of material with a pierced hole to install a pilot in the surrounding scrap to maintain as much dimensional stability as possible, and trimmed it off later.

Build in robustness, with thick shoes and robust guide pins, especially in older presses. Be aware of the flexing that may occur if the press has a large bolster opening. A few dollars more for upfront design and build to address robustness can save a lot of money over time.

2. Inspect Tools

Check critical tool component features as you build. Let’s say in development you realize a formed feature is not to print. The material springback is slightly greater than the engineering overbend calculation used in design—a common issue. You need to compensate and change the form punch and die. There is nothing worse than trying to fix this problem, only to find out later that the original tooling in the die was not made to print. Be sure to measure all tooling heights off a set point (usually a down stop or the die shoe itself) after assembly. This is the only way to discover height stack-up accumulations that will cause timing problems in your tool.

Do not assume just because the tooling was made by wire EDM that it will match the geometry used. You need to be 100 percent sure that the tooling meets the design and timing when basing your decisions on actions required to develop nonconforming features to meet print.

3. Stabilize the Process

The goal is to get the tool up and running at speed before tweaking the dimensions. The single focus here is stabilization.

I prefer to use micron sensors under the stripper plates. They measure the gap between the stripper and the sensor. If you pull a stamping slug and it lands between the product strip and die, this gap will change, and the micron sensor will shut you down.

Use at least two micron sensors in a 1-foot by 3-foot progressive tool. As the tool is running, you can view the reading at every hit. If you see a few 10 thousandths of an inch variation from hit to hit, you have a consistent, stable process. If you are seeing readings varying by a few thousandths, you have a problem. You must correct any instability before moving to the final adjustment phase.

4. Review Data and Part to Print

When reviewing the inspection data on the stamped part, start by looking at the range of the data. Do this by measuring the same feature over several parts. If the range is less than 50 percent of the specification tolerance, you generally have a process that is consistent and should meet a capability of 1.33—the general industry standard.

After you’ve determined the range, take a look at the mean reading of the data. You can use it as a guide to how much you’ll need to shift it to center the mean to the part specification. If the data has a range greater than the specification tolerances, how can you ever shift the mean to meet print? It is like trying to hit a moving target. You must first reduce the variation to an acceptable level, and then shift as needed. I have seen too many jobs pass first-piece approval, only to be shut down in production because parts are out of spec.

When in doubt, go back to the basics.